Numerous chemicals encountered in the environment alter male reproductive function. The rabbit studies are aimed at a better assessment of these changes after exposure to ethylene dibromide. Six semen samples will be collected weekly prior to exposure, then males will be dosed with EDB, and semen will be collected for 12 additional weeks. Females will be inseminated with semen from males before, and twice after, dosing to asses s fertilizing capacity of the sperm. The rat studies focus on the testicular lesion produced by boric acid. The rats consumed BA in the diet (9000ppm), and were sacrificed by anesthetic overdose at 4,7,10,14,21, and 28 days after start of exposure. Histologic analysis revealed that the release of mature spermatids was the process initially affected. Future studies will examine the extent of possible hormone changes in male rat exposed to dietary BA. Other rat studies have focused on the disposition of an active organophosphate intermediate in vivo. These studies (now complete) have found that the testis does not appear to accumulate this intermediate, which suggests that there is testicular production. We have shown that testicular cells in vitro can produce this active metabolite from tri-o- cresyl phosphate.